The present invention relates to a method of electrolytic surface treatment of aluminum supports for planographic printing plates, and more particularly to a method of keeping aluminum ion concentration constant in an electrolyte, the method being applicable to the electrolytes in an electrochemically roughening process, an anodic oxidation coating process, and a chemical etching process.
Various electrolytes have been conventionally used in the method for electrolytic surface treatment of aluminum supports for planographic printing plates. For example, nitric acid and hydrochloric acid are generally used in the electrochemically roughening process or sulfuric acid or the like is used in the anodizing process.
As a support used for a planographic printing plate, on the other hand, aluminum or an aluminum alloy is used. The surface shape or the like of the printing plates after treatment considerably vary according to the aluminum ion concentration in the foregoing electrolyte, and therefore it is important to keep the aluminum ion concentration constant in the electrolyte.
For example, in the case where graining is performed electrochemically using nitric acid, it is suitable not only to select the concentration of nitric acid to be 5 g/l-30 g/l, but also to select the aluminum ion concentration to be about 5 g/l-15 g/l so that the graining is most uniformly performed. Further, in the case where anodic oxidation is performed by using sulfuric acid, while it is suitable to select the concentration of sulfuric acid to be 100 g/l-300 g/l, the coating cannot be uniformly formed on the aluminum surface if the aluminum ion concentration exceeds 15 g/l.
In order to keep the aluminum ion concentration constant, there has been conventionally used a method in which an electrolyte in a system is discharged outside the system so as to keep the aluminum ion concentration constant in the electrolyte, or a method in which ion-exchange resin is used so as to absorb aluminum ions in an electrolyte to thereby keep the aluminum ion concentration constant in the electrolyte as disclosed in Japanese Patent Unexamined Publication No. 192300/82.
In the former method in which the electrolyte is discharged so as to control the aluminum ions outside the system, however, there has been a problem that the quantity of use of the electrolyte is excessively increased because the electrolyte is discharged together with aluminum ions, resulting in increased load for waste water treatment.
In the latter method in which aluminum ions are absorbed by ion-exchange resin so as to be discharged outside a system, on the other hand, there has been a problem that not only the rate of removal of the aluminum ions is low and therefore a large quantity of resin is required, but also it is necessary to exchange the resin about every three months, resulting in extreme increase in running cost.